Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World

by Mark Miodownik

the first stapler was hand-made for King Louis XV of France with each staple inscribed with his insignia. Who would have thought that staplers have royal blood?)

But there is also a scientific discipline especially dedicated to systematically investigating our sensual interactions with materials. This discipline, called psychophysics, has made some very interesting discoveries. For instance, studies of “crispness” have shown that the sound created by certain foods is as important to our enjoyment of them as their taste. This has inspired some chefs to create dishes with added sound effects. Some potato chip manufacturers, meanwhile, have increased not just the crunchiness of their chips but the noisiness of the chip bag itself.

Gold is another relatively soft metal, so much so that rings are very rarely made from pure gold metal because they quickly scratch. But if you alloy gold, by adding a small percentage of other metals such as silver or copper, you not only change the color of the gold—silver making the gold whiter, and copper making the gold redder—you make the gold harder, much harder.

In the case of gold alloys, you might wonder where the silver atoms go. The answer is that they sit inside the gold crystal structure, taking the place of a gold atom, and it is this atom substitution inside the crystal lattice of the gold that makes it stronger. Alloys tend to be stronger than pure metals for one very simple reason: the alloy atoms have a different size and chemistry from the host metal’s atoms, so when they sit inside the host crystal they cause all sorts of mechanical and electrical disturbances that add up to one crucial thing: they make it more difficult for dislocations to move. And if dislocations find it difficult to move, then the metal is stronger, since it’s harder for the metal crystals to change shape. Alloy design is thus the art of preventing the movement of dislocations.

A crystal of aluminum oxide is colorless if pure but becomes blue when it contains impurities of iron atoms: it is the gemstone called sapphire. Exactly the same aluminum oxide crystal containing impurities of chromium is the gem called ruby.

Copper is a weak metal, but naturally occurring and easy to smelt. Bronze is an alloy of copper, containing small amounts of tin or sometimes arsenic, and is much stronger than copper. So, if you had copper and you knew what you were doing, for very little extra effort you could create weapons and razors ten times stronger and harder than copper. The only problem is that tin and arsenic are extremely rare. Elaborate trade routes evolved in the Bronze Age to bring tin from places such as Cornwall and Afghanistan to the centers of civilization in the Middle East for precisely this reason.

Steel, the alloy of iron and carbon, is even stronger than bronze, with ingredients that are much more plentiful: pretty much every bit of rock has some iron in it, and carbon is present in the fuel of any fire. Our ancestors didn’t realize that steel was an alloy—that carbon, in the form of charcoal, was not just a fuel to be used for heating and reshaping iron but could also get inside the iron crystals in the process. Carbon doesn’t do this to copper during smelting, nor to tin or bronze, but it does to iron. It must have been incredibly mysterious—and only now with a knowledge of quantum mechanics can we truly explain why it happens (the carbon in steel doesn’t take the place of an iron atom in the crystal, but is able to squeeze in between the iron atoms, creating a stretched crystal).

If iron becomes alloyed with too much carbon—if, for instance, it contains 4 percent carbon instead of 1 percent carbon—then it becomes extremely brittle and essentially useless for tools and weapons. This is a major obstacle because inside a fire there is rather a lot of carbon around. Leave the iron in too long, or allow it to become liquid in the fire, and a huge amount of carbon enters the metal crystals, making the alloy very brittle. Swords made from this high-carbon steel snap in battle.

for thousands of years the ritual of shaving began with the process of stropping: the act of sharpening the blade by playing it back and forth along a length of leather. You might think it not credible that a material as soft as leather can sharpen steel, and you would be right. It is the fine ceramic powder that is impregnated in the leather strop that does the sharpening. Traditionally a mineral called jewelers’ rouge was used, but these days diamond powder is more common. The act of running the steel along the strop, in a flip-flop manner, causes the blade to meet the hard particles of diamond that are in the powder, which remove very small amounts of metal in the collision, restoring the delicately fine cutting edge.

Paper yellows with age for two reasons. If it is made from cheap, low-grade mechanical pulp, it will still contain some lignin. Lignin reacts with oxygen in the presence of light to create chromophores (meaning, literally, “color-carriers”), which turn the paper yellow as they increase in concentration.

The aging process also results in the formation of a wide range of volatile (meaning that they evaporate easily) organic molecules, which are responsible for the smell of old paper and books. Libraries are now actively researching the chemistry of book smell to see if they can use it to help them monitor and preserve large collections of books. Although it is a smell of decay, to many it is nevertheless perceived to be a pleasant one.

That paper, a much softer material than either stone or wood, won out as the guardian of the written word is a remarkable materials story. It is the thinness of paper that proves to be one of its great advantages, allowing it the flexibility to survive continuous handling, but when stacked together in book form becoming stiff and strong—essentially a re-formed block of wood. With the use of hard covers to hold it all together, the book is a fortress for words for thousands of years.

Paper’s mechanical properties lend themselves to folding and bending. The cellulose fibers of which it is made can be partially snapped in the area of maximum bend, allowing a permanent crease to form, while sufficient fibers remain intact for the material not to crack and fall apart. Indeed, in this state it pretty much maintains its ability to resist being pulled apart, but it can also be torn easily and accurately along the crease if a point of weakness—a small, initial tear—is opened up. This winning combination of mechanical properties allows it to assume the shape of any object through creasing and folding—hence the art of origami.

in the case of paper: it is itself a fully fledged industrial product, and quite an environmentally costly one too. The impact in terms of energy usage of a single-use paper bag has been found to be greater than that of a plastic bag.

concrete doesn’t dry out. Quite the opposite, water is an ingredient of concrete. When concrete sets, it is reacting with the water, initiating a chain of chemical reactions to form a complex microstructure deep within the material, so that this material, despite having a lot of water locked up inside it, is not just dry but waterproof.

In the case of cement, the skeleton is made up of calcium silicate hydrate fibrils, which are crystal-like entities that grow from the calcium and silicate molecules, now dissolved in the water, in a way that appears almost organic (see the picture below). So the gel that forms inside cement is constantly changing as the solid internal skeleton grows and further chemical reactions take place. A sketch of calcium silicate fibrils growing inside the setting cement. As the fibrils grow and meet, they mesh together, forming bonds and locking in more and more of the water, until the whole mass transforms from a gel to a solid rock. These fibrils will bond not only to each other but also to other rocks and stones, and this is how cement turns into concrete. Cement is used to bond together bricks to make houses and stones to make monuments, but in both these cases it is wedged between the cracks as the minority component, an urban glue. When it is made into concrete by mixing it with small stones, which play the role of tiny bricks, it fulfils its potential to become a structural material.

if you add too much water there won’t be enough calcium silicate from the cement powder to react with, and so water will be left over within the structure, which makes it weak. Similarly, if you add too little water there will be unreacted cement left over, which again weakens the structure. It is usually human error of this sort that proves the undoing of concrete. Such poor concrete can go undiscovered but then lead to catastrophe many years after the builders have departed. The extent of the devastation due to the 2010 earthquake in Haiti was blamed on shoddy construction and poor-quality concrete: an estimated 250,000 buildings collapsed, killing more than 300,000 people, and making a million more homeless. What is worse is that Haiti is by no means unusual. Such concrete time bombs are scattered throughout the world.

The other way of breaking a material is to create a crack through it, which is how a glass or a tea cup breaks: unable to flow in order to accommodate the stress that is pulling it apart, a single weakness in this type of material compromises the integrity of the whole, and it splits or shatters. This is how concrete breaks, which was a big headache for the Romans. The Romans never solved this problem and so only used concrete in situations where it was being compressed rather than stretched, such as in a column, dome, or the foundations of a building, where every part of the concrete was being squeezed together by the weight of the structure. Under such compression, concrete remains strong even when cracks form.

Self-healing concrete has these bacteria embedded inside it along with a form of starch, which acts as food for the bacteria. Under normal circumstances these bacteria remain dormant, encased by the calcium silicate hydrate fibrils. But if a crack forms, the bacteria are released from their bonds, and in the presence of water they wake up and start to look around for food. They find the starch that has been added to the concrete, and this allows them to grow and replicate. In the process they excrete calcite, a form of calcium carbonate. This calcite bonds to the concrete and starts to build up a mineral structure that spans the crack, stopping further growth of the crack and sealing it up. It’s the sort of idea that might sound good in theory but never work in practice. But it does work. Research now shows that cracked concrete that has been prepared in this way can recover 90 percent of its strength thanks to these bacteria. This self-healing concrete is now being developed for use in real engineering structures.

Another type of concrete with a living component is called filtercrete. This is a concrete that has very particular porosity, such that it allows naturally occurring bacteria to colonize it. The pores in the concrete also allow water to flow through it, reducing the need for drains, while the bacteria inside the concrete purify the water by decomposing oils and other contaminants.

And there is also now a textile version of concrete called concrete cloth. This material comes in a roll and needs only water to be added for it to harden into any shape you like. Although this material has great sculptural potential, perhaps its biggest application may be in disaster zones, where tents made in situ from rolls of concrete dropped from the air can create a temporary city in a matter of days, one that will keep out the rain, wind, and sun for years while rebuilding efforts continue.

Many new versions of concrete have been invented to refresh its aesthetic appeal. The latest is self-cleaning concrete, which contains titanium dioxide particles. These sit on its surface but are microscopic and transparent, so it looks no different. However, when they absorb UV light from the sun, the particles create free radical ions, which break down any organic dirt that comes into contact with them. The remains are washed away by the rain or blown away by the wind. A church in Rome called Dives in Misericordia has been constructed with such self-cleaning concrete. In fact, the titanium dioxide does more than clean the concrete: it can also reduce the level of nitrogen oxide in the air, produced by cars, like a catalytic converter. Several studies have shown that this works, and open up the possibility that in the future buildings and roads may not be purely passive; they may purify the air much like plants.

But cocoa butter is a special fat for many reasons. For one, it melts at body temperature, meaning that it can be stored as a solid but becomes a liquid when it comes into contact with the human body. This makes it ideal for lotions. Moreover, it contains natural antioxidants, which prevent rancidity, so it can be stored for years without going off (compare that to butter made from milk, which has a shelf life of only a few weeks). This is good news for face cream makers but also for chocolate manufacturers. Cocoa fat has another trick up its sleeve: it forms crystals, and these are what give chocolate bars their mechanical strength.

if you melt some chocolate and then let it cool down, you will almost certainly form Types III and IV crystals—this chocolate feels soft to the touch, has a matte finish, and melts easily in the hand. These crystals will transform into the more stable Type V over time, but on the way they will eject some sugar and fat, which will appear as white powder on the surface of the chocolate—called bloom.

When you put pure dark chocolate into your mouth and sense it start to liquefy, what you are feeling are the Type V cocoa butter crystals that are holding the chocolate together starting to wobble. If they have been cared for properly, they will have spent their entire life at temperatures below 18°C.

Dark chocolate usually contains 50 percent cocoa fat and 20 percent cocoa nut powder (referred to as “70 percent cocoa solids” on the packaging). Almost all the rest is sugar. Thirty percent sugar is a lot. It’s the equivalent of putting a spoonful of sugar in your mouth. Nevertheless dark chocolate isn’t overly sweet; sometimes it’s not sweet at all. This is because at the same time that the sugars are released by the melting cocoa butter, so are chemicals known as alkaloids and phenolics from the cocoa powder. These are molecules such as caffeine and theobromine, which are extremely bitter and astringent. They activate the bitter and sour taste receptors and complement the sweetness of the sugar. Balancing these basic tastes to give the chocolate a rounded flavor is the first task of the chocolatier. The addition of salt as a flavor enhancer, as well as adding another dimension to modern chocolates, has in turn led to chocolate being used as an ingredient in savory dishes: it is the basis of the Mexican dish pollo con mole, which is chicken cooked in dark chocolate.

However, cooked chocolate tastes different from eating chocolate for another reason. Although basic taste is generated on the tongue by the taste buds, which distinguish between bitter, sweet, salty, sour, and umami (meaty or savory), most flavor is experienced through smell. It is the smell of chocolate from within your own mouth that is responsible for its complex taste. When you cook chocolate, many of its flavor molecules evaporate or are destroyed by the cooking. This is a problem not just for hot chocolate but also for coffee and tea. It is why you need to drink those drinks within minutes of being brewed, otherwise the flavor disappears into the air. It is also why you lose much of your sense of taste when you have a cold—because the smell receptors in the nose are covered by mucus.

The reason why any sugar molecule—whether in a cocoa bean or a pan or anywhere else—turns brown when heated is to do with the presence of carbon. Sugars are carbohydrates, which is to say that they are made of carbon (“carbo-”), hydrogen (“hydr-”), and oxygen (“-ate”) atoms. When heated, these long molecules disintegrate into smaller units, some of which are so small that they evaporate (which accounts for the lovely smell). On the whole, it is the carbon-rich molecules that are larger, so these get left behind, and within these there is a structure called a carbon–carbon double bond. This chemical structure absorbs light. In small amounts it gives the caramelizing sugar a yellow-brown color. Further roasting will turn some of the sugar into pure carbon (double bonds all round), which creates a burnt flavor and a dark-brown color. Complete roasting results in charcoal: all of the sugar has become carbon, which is black.

the Maillard reaction. This is when a sugar reacts with a protein. If carbohydrates are the fuel of the cellular world, proteins are the workhorses: the structural molecules that build cells and all their internal workings. Seeds (in the form of nuts or beans) must contain all of the proteins needed to get the cellular machinery of a plant up and running, so there is plenty of protein in the cocoa beans. When subjected to temperatures of 160°C and above, these proteins and carbohydrates start to undergo Maillard reactions, reacting with the acids and esters (produced by the earlier fermentation process) and resulting in a huge range of smaller flavor molecules. It is no exaggeration to say that without the Maillard reaction the world would be a much less delicious place: it is the Maillard reaction that is responsible for the flavor of bread crust, roasted vegetables, and many other roasted, savory flavors. In this case, the Maillard reaction is responsible for the nutty, meaty flavors of chocolate, while also reducing some of the astringency and bitterness.

When European explorers got hold of the drink in the seventeenth century, they exported it to coffee houses, where it competed with tea and coffee to be the beverage of choice of Europeans—and lost. What no one had really mentioned was that chocolatl means “bitter water,” and even though it was sweetened with the new cheap sugar flooding in from the slave-run plantations of Africa and South America, it was also a gritty, oily, and heavy drink, because 50 percent of the cocoa bean is cocoa fat. This is how it remained for another two hundred years: an exotic drink, notable but not terribly popular.

These days the type of milk added to chocolate varies widely throughout the world, and this is the main reason that milk chocolate tastes different from country to country. In the USA the milk used has had some of its fat removed by enzymes, giving the chocolate a cheesy, almost rancid flavor. In the UK sugar is added to liquid milk, and it is this solution, reduced to a concentrate, that is added to the chocolate, creating a milder caramel flavor. In Europe powdered milk is still used, giving the chocolate a fresh dairy flavor with a powdery texture. These different tastes do not travel well. Despite globalization, the preferred taste of milk chocolate, once acquired, remains surprisingly regional.

There are plenty of people, including myself, who are addicted to eating chocolate, and the reason may not just be its taste. It also contains psychoactive ingredients. The most familiar one is caffeine, which is present in small proportions in the cocoa bean, and so ends up in the chocolate via the cocoa powder. The other psychoactive ingredient is theobromine, which is a stimulant and antioxidant, like caffeine, but is also highly toxic to dogs. Many dogs die every year from eating chocolate, mainly around Easter and Christmas. Theobromine’s effect on humans appears to be much milder, and the stimulant levels in chocolate are small when compared to coffee and tea, so even if you eat a dozen chocolate bars every day, it is only equivalent to drinking one or two cups of strong coffee. Chocolate also contains cannabinoids, which are the chemicals responsible for the high experienced from smoking dope. But again the percentages are tiny,

This leaves another possibility to explain chocolate addiction. Rather than its being a chemical effect, it may be that the sensory experience of eating chocolate is itself addictive. Chocolate is like no other food. When chocolate melts in the mouth it suddenly releases a wild and complex, sweet and bitter cocktail of flavors within a warm rich liquid. It is not just a flavor but an entire oral experience. It is soothing and comforting, but it’s also exciting and—not to put too fine a point on it—seems to satisfy more than a physical hunger.

if solid chocolate is exposed to temperatures above 20°C, as a result perhaps of being left in the sun or in a hot car, it undergoes fundamental changes of structure. The changes can be spotted immediately because they result in “bloom”: fat and sugars migrate to the surface of the chocolate and form a whitish crystalline powder, often with a river mark pattern.

When light from the sun enters the Earth’s atmosphere, it hits all sorts of molecules (mostly nitrogen and oxygen molecules) on its way to Earth and bounces off them like a pinball. This is called scattering, which means that on a clear day, if you look at any part of the sky, the light you see has been bouncing around the atmosphere before coming into your eye. If all light was scattered equally, the sky would look white. But it doesn’t. The reason is that the shorter wavelengths of light are more likely to be scattered than the longer ones, which means that blues get bounced around the sky more than reds and yellows. So instead of seeing a white sky when we look up, we see a blue one.

These days it is hard to believe that anyone could make fundamental chemical discoveries in their shed. But in the late nineteenth century, the beginning of the golden age of chemical engineering, a growing understanding of chemistry coincided with entrepreneurial opportunities for making money out of the invention of new materials. It was also easy and cheap to get hold of chemicals, sales of which were mostly unregulated. Many inventors were operating from their homes—and, in the case of Goodyear, from debtors’ prison. Once his rubber proved itself, the demand for the protection, comfort, and flexibility of this type of material grew.

Unlike previous methods, formaldehyde preserved the tissue in such a way as to give the corpse an almost lifelike appearance, and it soon became the method of choice. Lenin, Kemal Atatürk, and Diana, Princess of Wales, were all embalmed with formaldehyde. These days, a new technique called plastinization has been developed by Gunther von Hagens. This involves the removal of water and fat (such as lipids) from the body, and their replacement, using a vacuum technique, with silicone rubber and epoxy resin, a hugely versatile material that is used in all sorts of paints and adhesives and flexible products. Like formaldehyde, this produces a lifelike appearance, but because of the stiffness of the plastics used, the bodies can be set into lifelike poses. An exhibition of these preserved and posed bodies, Body Worlds, has been touring the world since 1995 and has been seen by millions.

The lack of glass technology in the East meant that, despite their technical sophistication, they never invented the telescope nor the microscope, and had access to neither until Western missionaries introduced them. Whether it was the lack of these two crucial optical instruments that prevented the Chinese from capitalizing on their technological superiority and instigating a scientific revolution, as happened in the West in the seventeenth century, is impossible to say. What is certain, though, is that without a telescope you can’t see that Jupiter has moons, or that Pluto exists, or make the astronomical measurements that underpin our modern understanding of the universe. Similarly, without the microscope, it is impossible to see cells such as bacteria and to study systematically the microscopic world, which was essential to the development of medicine and engineering.

Within every atom there is a central nucleus, which contains protons and neutrons, surrounded by varying numbers of electrons. The size of the nucleus and the individual electrons is tiny compared to the overall size of the atom. If an atom were the size of an athletics stadium, the nucleus would be the size of a pea at its center, and the electrons would be the size of grains of sand in the surrounding stands. So within all atoms—and indeed all matter—there is a majority of empty space.

This idea of electrons not being able to move between rows (or energy states, as they are called) unless the energy exactly matches is the theory that governs the atomic world, called quantum mechanics. The gaps between rows correspond to specific quantities of energy, or quanta. The way these quanta are arranged in glass is such that moving to a free row requires much more energy than is available in visible light. Consequently, visible light does not have enough energy to allow the electrons to upgrade their seats and has no choice but to pass straight through the atoms. This is why glass is transparent. Higher-energy light, on the other hand, such as UV light, can upgrade the electrons in glass to the better seats, and so glass is opaque to UV light. This is why you can’t get a suntan through glass, since the UV light never reaches you. Opaque materials like wood and stone effectively have lots of cheap seats available and so visible light and UV are easily absorbed by them.

Whether the relationship between glass technology and the seventeenth-century scientific revolution really is a simple case of cause and effect is an open question. It seems more likely that glass was a necessary condition rather than the reason for it. However, there is no doubt that glass was largely ignored in the East for a thousand years. And during this time, glass revolutionized one of Europe’s most treasured customs.

The move to serving beer in glasses had another unexpected side effect. According to the UK government, more than five thousand people are attacked with glasses and bottles every year, costing the health service more than £2 billion to surgically repair the injured. Although many alternative plastic materials for serving beer in bars and pubs have been tried, materials which are both transparent and tough, they have never gained acceptance. Drinking beer from a plastic cup is a completely different experience to drinking from a glass. Not only does plastic taste different, but it also has a lower thermal conductivity, a property that makes it feel warmer than glass, reducing the satisfaction of drinking an ice-cold beer. Plastic is also much softer than glass, so plastic beer cups soon become tarnished, scratched, and opaque. This masks the clarity of the beer but it also affects our perception of the cleanliness of the vessel.

I was constantly being dragged to museums as a child, to the national this or that, and without exception I was bored in all of them. I tried to do what the adults did and walk around in ponderous silence or ruminate in front of a painting or a sculpture, but it didn’t work for me. I got nothing out of it as far as I could fathom. It was very different when we visited the crown jewels.

Transporting the Cullinan diamond back to Britain posed an enormous security challenge for its owners, since the discovery of the largest ever rough diamond had been widely reported in the newspapers. Notorious criminals like Adam Worth, the inspiration for Sherlock Holmes’s nemesis, Moriarty, who had already managed to steal a whole shipment of diamonds, were perceived to be a real threat. In the end, a plan worthy of Sherlock Holmes was hatched and executed. A decoy stone was dispatched on a steamboat under high security while the real one was sent by post in a plain brown paper box. The ruse worked precisely because of another remarkable attribute of diamond: being composed solely of carbon, it is extremely light.

Take a look at the graphite of a pencil and you will see that it is dark gray and shiny like a metal. For thousands of years it was mistaken for lead and was referred to as “plumbago,” or “black lead,” hence the use of the term “lead” to refer to the graphite used in a pencil. The confusion is understandable since they are both soft metals (although these days we call graphite a semi-metal). Plumbago mines became more and more valuable as new uses were found for graphite, such as the discovery that it was the perfect material to cast cannon and musket balls. In seventeenth- and eighteenth-century Britain the material became so expensive that thieves took to digging secret tunnels into the mines, or working in the mines and secreting the plumbago about their person.

The type of coal most revered for its aesthetic appeal is that derived from fossilized monkey puzzle trees. It is hard, can be carved and polished to a brilliant finish, and has a beautiful dark black luster. It is sometimes called black amber because it has similar triboelectric properties to amber: the ability to generate static charge and make hair stand on end. We know it more commonly as jet. It was made fashionable in Britain in the nineteenth century by Queen Victoria, who mourned the death of her consort Prince Albert by wearing black clothes and jet jewelry for the rest of her life. There was subsequently such a popular demand for jet from the rest of the British Empire that overnight the population of the Yorkshire town of Whitby, where Bram Stoker later wrote his Gothic masterpiece Dracula, stopped using the large local deposits of jet for fuel and became famous for producing the jewelry of lament and sorrow.

Paper cups may seem sustainable because paper is recyclable, but the wax coating required to make them waterproof makes this almost impossible. For real sustainability, we must look to ceramics.

whole towns and cities are built from the stuff: the common house brick is essentially a form of terra cotta. The big problem with terra cotta ceramics, though, is that they never get rid of all the holes, and so never become fully dense. This is fine for house bricks, which only need to be fairly strong, and once cemented in place will not be bashed around or heated and cooled repeatedly. But it is a disaster for a cup or a bowl, which will have a thin body but be expected to withstand the rigors of the kitchen. They just don’t last: one small knock and the cracks start to grow from the pores and never stop.

The invention in 1840 of an alloy comprised mainly of silver, tin, and mercury, called amalgam, was the turning point. In its preliminary form, amalgam is a liquid metal at room temperature because of its mercury content. However, when it’s mixed with its other components, a reaction takes place between the mercury and the silver and tin that results in a new crystal, which is fully solid, hard-wearing, and tough. This miracle material could be squirted into a cavity while it was liquid, and then left to set hard. As it solidifies, the amalgam also expands slightly, wedging the filling into the cavity so that it becomes strongly mechanically bonded to the tooth.

composite fillings are a combination of a strong transparent plastic and a silica powder that makes them hard and resistant to wear, while also matching the color of teeth better than the amalgams. These fillings, like amalgam, are molded into the cavity while liquid. Once they’re in place, though, a small ultraviolet light is introduced into the mouth, which activates a chemical reaction within the resin that sets it hard almost instantly.